Electrical stimulation of excitable body tissue is used as a method for treating various pathological conditions. Therapeutic stimulation generally requires making an electrical contact between excitable tissue and an electrical pulse generator through use of one or more stimulation leads. Various lead systems and various techniques for implanting these lead systems in contact with excitable body tissue, and particularly the heart, have been developed.
In order to achieve cardiac pacing, sensing, cardioversion and/or defibrillation at different locations in the heart, various types of cardiac leads have been developed including epicardial leads, endocardial leads, and coronary vein leads. A transvenous endocardial lead establishes electrical contact between an electrical pulse generator, such as a pacemaker or implantable cardioverter defibrillator, and the endocardial surface of the heart, typically in a right heart chamber. Endocardial leads, and cardiac leads in general, may be held in place by passive fixation mechanisms, such as tines that interact with the ventricular trabeculae, or active fixation mechanisms, such as a helix. A coronary vein lead may be passed through a venous pathway, into the right atrium, through the coronary sinus ostium and ultimately to a location deep in the cardiac veins. Contact is made with the epicardial surface of the left atrium or left ventricle for delivering stimulation or sensing cardiac signals in the left heart chambers. Epicardial leads are also known in the art and generally require a thoracotomy for placement on the epicardial surface of a heart chamber.
The safety, efficacy and longevity of an electrical pulse generator depends, in part, on the performance of the associated cardiac lead(s) used in conjunction with the pulse generator. Various properties of the lead, the electrodes and the tissue interfacing with an electrode will result in a characteristic impedance, stimulation threshold and sensing threshold.
Stimulation threshold is the energy required in a stimulation pulse to depolarize, or “capture,” the heart tissue. A relatively high impedance and low threshold is desired to minimize the current drawn from a pulse generator battery in delivering a stimulation pulse. Maximizing the useful life of the pulse generator battery is important since a surgical procedure is required to replace the pulse generator once the battery has reached the end of its useful life.
One factor that can affect the stimulation threshold, particularly during the first several weeks after implantation of a lead, is the natural immunological response of the body to the lead as a foreign object. The presence of the lead activates the immunologic response, which ultimately results in fibrotic encapsulation of the lead and its electrodes. Since fibrotic tissue is not excitable tissue, an elevated stimulation threshold can persist due to the degraded electrical properties of the electrode-tissue interface.
To reduce the inflammatory response, medical leads that elute an anti-inflammatory steroid have been developed. Steroid eluting leads are described in U.S. Pat. No. 4,506,680 issued to Stokes and related Medtronic U.S. Pat. Nos. 4,577,642, and 4,606,118. Steroid eluting leads may require a monolithic controlled release device (MCRD) to contain the steroid and to thereafter slowly leach out the water soluble steroid into the surrounding tissue. A method for applying a steroid directly to the surface of an electrode is disclosed in U.S. Pat. No. 5,987,746 issued to Williams. Advantages of this method include elimination of additional structures for carrying the steroid and the presentation of the steroid directly at the tissue-electrode interface.
One limitation of a steroid eluting electrode or MCRD, however, is that a relatively limited volume of tissue is treated by the eluting drug since the drug is presented only at the endocardial or epicardial surface. Other devices have been proposed which allow the delivery of a drug to a potentially larger volume of tissue by actually penetrating the tissue rather than relying on diffusion of the drug from the tissue surface. Drug delivery catheters may incorporate a drug dispensing needle or helix that penetrates a targeted tissue for delivering a drug or fluid. Catheters that may be used to deliver a fluid or drug into the myocardium are disclosed in U.S. Pat. No. 6,102,887 issued to Altman and U.S. Pat. No. 5,431,649 issued to Mulier et al.
Drug delivery catheters may include an electrode to allow sensing or stimulation of the myocardium. An implantable pacing lead having an active fixation electrode with a stylet introduced, anti-inflammatory drug delivery system is disclosed in U.S. Pat. No. 5,447,533 issued to Vachon et al. A delivery system for delivering a therapeutically effective amount of a genetic material to an identified cardiac location adjacent an atrial or ventricular electrode is disclosed in PCT Patent Publication WO 98/02040 issued to Stokes et al, incorporated herein by reference in its entirety. This delivery system may combine a pacing lead and a delivery catheter. Other implantable leads with drug delivery capabilities are disclosed in U.S. Pat. No. 4,360,031 to White, and U.S. Pat. No. 5,496,360 to Hoffman.
Advancements in gene therapies and cellular modifications through the delivery of proteins, peptides or even cell delivery, such as stem cell delivery, offer opportunities to alter the properties of tissue to further improve the benefit of a delivered stimulation therapy or improve the ability to sense cardiac signals. Genetic or biologic agents may be used to alter ion channel activity or protein expression at the cellular level. Potential benefits include decreased inflammatory response, increased tissue conductivity for reduction of stimulation thresholds or upregulation of ion channels for increasing membrane potentials to allow better sensing. For example, upregulation of ion channels could enhance cardiac P-waves or R-waves allowing them be more easily sensed by a pacemaker or other cardiac monitor. In particular, cardiac fast sodium channels are responsible for the fast upstroke of the action potential in myocardial cells (Fozzard, et al., Circ. Res. 1995, 56:475–485). A human cardiac voltage-dependent sodium channel, hH1, has been cloned, sequenced, and functionally expressed (Gellens, et al., Proc. Natl. Acad. Sci. USA, 1992, 89:554–558). Alteration of myocardial conductivity may be possible through delivery of proteins that alter cellular electrical coupling. The gap junction protein Connexin43 has been found to play an important role in ventricular conduction (Guerrero P A et al., J. Clin. Invest. 1997, 99:1991–1998).
Because locally effective doses of a pharmacologic, genetic, or biologic agent may be toxic when given systemically, it is desirable to provide a method for delivering an agent locally at a targeted tissue site. Drug-eluting electrodes may be limited to treating only a relatively small volume of tissue at an electrode-tissue interface. The pharmacological effect is in part limited by the kinetics of the drug leaving the electrode or lead. Furthermore, because biologic and genetic agents may have a limited shelf life, unique storage requirements such as requiring refrigeration, and may not tolerate sterilization procedures, it is not desirable to package a lead having drug eluting capabilities with the biologic or genetic agent already incorporated therein. Other medical leads having drug dispensing capabilities may require additional components that increase the size, stiffness or complexity of the lead. There is a need, therefore, for a fluid delivery system wherein certain dispensing components, though compatible with an implantable lead, are not integral with the implantable lead so that these components need not be assembled into the lead prior to implant and, once a fluid agent has been delivered, these components, which are no longer needed, may be removed from the patient's body leaving the implanted lead behind.